1. Field of the Invention
The present invention relates to diffractive optical elements, and more particularly to a diffractive optical element having such a grating structure that rays of a plurality of wavelengths or rays of a specific wavelength band concentrate at a specific order (a design order) of diffraction, and to an optical system having the diffractive optical element.
2. Description of Related Art
Heretofore, as one of the methods of correcting chromatic aberration of an optical system, there is known a method of combining two glass (lens) materials which differ in dispersion.
In contrast to the method of reducing chromatic aberration by combining glass materials, there is known another method, which is disclosed in the optical literature, such as xe2x80x9cInternational Lens Design Conference (1990)xe2x80x9d, SPIE Vol. 1354, etc., and the specifications of Japanese Laid-Open Patent Applications No. HEI 4-213421 and No. HEI 6-324262 and U.S. Pat. No. 5,044,706. In the case of that method, chromatic aberration is corrected by means of a diffractive optical element which is provided with a diffraction grating for a diffracting action and is disposed on a lens surface or a part of an optical system. That method is based on a physical phenomenon that the direction in which chromatic aberration arises for a ray of light of a reference wavelength becomes opposite between a refractive surface and a diffractive surface in an optical system.
Further, the diffractive optical element of such a type can be arranged to produce an advantageous correcting effect, like an aspheric lens, on the aberration by varying the period of a periodic structure of its diffraction grating.
Here, compared with a refracting action of rays of light, while one ray of light remains one even after refraction at a lens surface, one ray of light is split into rays of a plurality of orders after diffraction at a diffractive surface.
Therefore, in using a diffractive optical element for a lens system, it is necessary to decide the grating structure in such a way as to cause a light flux of a useful wavelength region to concentrate at a specific order of diffraction (hereinafter referred to as a design order). With the light flux concentrating at the specific order, rays of diffraction light other than the light flux of the specific order have a low degree of intensity. When the intensity becomes zero, the rays of diffraction light would not exist.
In order to attain the above-stated feature, the diffraction efficiency of a ray of light of the design order must be sufficiently high. Further, in a case where there are some rays of light having diffraction orders other than the design order, these rays are imaged in a place different from the imaging place of the ray of light of the design order, and thus appear as flare light.
For an optical system using a diffractive optical element, therefore, it is important to pay sufficient heed to the spectral distribution of diffraction efficiency at the design order and the behavior of rays of diffraction light of orders other than the design order.
FIG. 12 shows a case where a diffractive optical element 1, which has a diffraction grating 3 and is composed of one layer on a base plate 2, is formed on a surface of an optical system. In this case, diffraction efficiency for a specific order of diffraction is obtained as shown in FIG. 13, which shows the characteristic of the diffraction efficiency. This diffractive optical element is made of plastic material which is PMMA (nd=1.4917 and xcexdd=57.4). The grating thickness d is set at 1.07 xcexcm. In FIG. 13, the abscissa axis of a graph indicates wavelength (nm) and the ordinate axis indicates the diffraction efficiency (%).
The diffractive optical element 1 is designed to have the diffraction efficiency become highest at the first order of diffraction (shown in a full line curve in FIG. 13) in the useful wavelength region (a wavelength of 530 nm and thereabout). In other words, the design order of the diffractive optical element 1 is the first order.
Further, FIG. 13 shows also the diffraction efficiency of a diffraction order near the design order, i.e., zero-order light and second-order light (first order +first order). As shown in FIG. 13, at the design order, the diffraction efficiency becomes highest at a certain wavelength (hereinafter referred to as a xe2x80x9cdesign wavelengthxe2x80x9d) and gradually decreases at other wavelengths. In this case, the design wavelength xcex is set at 530 nm. The lower portion of the diffraction efficiency obtained at the design order becomes diffraction light of other orders and comes to appear as flare light. Further, in a case where the optical system is provided with a plurality of diffractive optical elements, a drop in diffraction efficiency at wavelengths other than the design wavelength eventually causes a decrease in transmission factor.
A diffractive optical element having the structure capable of lessening the drop in diffraction efficiency is disclosed in Japanese Laid-Open Patent Application No. HEI 9-127321. More specifically, the diffractive optical element disclosed is constructed by laminating on a base plate a plurality of layers of different materials and forming a relief pattern of diffraction grating on an interface between the layers of different materials.
The diffractive optical element of the kind having a grating structure formed by laminating a plurality of layers on a base plate can be formed to have a high degree of diffraction efficiency by combining component layers in various manners. Some of such combinations, however, make the thickness of grating of the diffractive optical element thicker than in the case of an ordinary element composed of a single layer, because materials forming diffraction gratings at their boundary faces cannot be otherwise arranged to have a sufficiently large difference in refractive index. Such a combination that causes an increase in thickness of grating presents a problem in a case where the refractive index is caused to vary by temperature variations to lower the diffraction efficiency.
In the event of temperature variations, some of such combinations thus cause the diffraction efficiency of the diffractive optical element of this kind to become lower than that of a diffractive optical element composed of a single layer in a conventional manner.
It is an object of the invention to provide a diffractive optical element, or an optical system having the diffractive optical element, which is arranged to be capable of keeping its diffraction efficiency not much degraded by changes of refractive index resulting from temperature variations.
To attain the above object, in accordance with a first aspect of the invention, there is provided a diffractive optical element having a diffraction grating formed at an interface between different materials, in which a total sum of values obtained by multiplying rates of change in refractive index due to temperature variations of the materials by a grating thickness of the diffraction grating is smaller than a useful wavelength.
In accordance with a second aspect of the invention, there is provided a diffractive optical element having diffraction gratings formed by a plurality of layers made of at least two kinds of materials of different dispersions to enhance diffraction efficiency of a specific order (design order) over an entire useful wavelength region, in which a total sum of values obtained by multiplying rates of change in refractive index due to temperature variations of the materials forming the respective layers by grating thicknesses of the respective diffraction gratings is smaller than a useful wavelength.
In accordance with a third aspect of the invention, there is provided a diffractive optical element having a first diffraction grating surface formed at a boundary between first and second layers made of materials of different dispersions and a second diffraction grating surface formed at a boundary between the second layer and air to enhance diffraction efficiency of a design order (specific order) over an entire useful wavelength region, in which, letting rates of change in refractive index due to temperature variations of the materials of the first and second layers be denoted by dn1/dt and dn2/dt, respectively, a grating thickness of a first diffraction grating formed at the first layer be denoted by d1, a grating thickness of a second diffraction grating formed at the second layer having the first and second diffraction grating surfaces be denoted by d2, and a value obtained by multiplying the rates of change in refractive index due to temperature variations by the grating thicknesses of the first and second diffraction gratings be denoted by xcfx86t, the value xcfx86t being expressed as follows:
xcfx86t=(dn1/dt)d1xe2x88x92(dn2/dt)d2
and letting an amount of temperature variations be denoted by xcex94t, the design order be denoted by m, and a useful wavelength be denoted by xcex0, the diffractive optical element satisfies the following condition:
|xcfx86txc2x7xcex94t| less than mxc2x7xcex0/4.
In accordance with a fourth aspect of the invention, there is provided a diffractive optical element having diffraction gratings formed by a plurality of layers made of at least two kinds of materials of different dispersions to enhance diffraction efficiency of a design order (specific order) over an entire useful wavelength region, in which, letting a rate of change in refractive index due to temperature variations of the material of the L-th layer be denoted by dnoL/dt, a grating thickness of the diffraction grating formed on the L-th layer be denoted by dL, and a value obtained by multiplying the rate of change in refractive index due to temperature variations by the grating thickness of each diffraction grating be denoted by xcfx86t, the value xcfx86t being expressed as follows:
xcfx86t=(dno1/dt)d1xc2x1(dno2/dt)d2xc2x1 . . . xc2x1(dnoL/dt)dL
and letting an amount of temperature variations be denoted by xcex94t, the design order be denoted by m, and a useful wavelength be denoted by xcex0, the diffractive optical element satisfies the following condition:
|xcfx86txc2x7xcex94t| less than mxc2x7xcex0/4.
Further, in the diffractive optical element according to one of the first to fourth aspects, the plurality of diffraction gratings include a diffraction grating in which a grating thickness thereof within one period monotonously decreases in one direction and a diffraction grating in which a grating thickness thereof within one period monotonously increases in the one direction.
In particular, among the plurality of layers, if the diffractive optical element has an optical shape (optical path length) formed to monotonously (monotonically) increase the grating thickness within one period, at least one dispersion of the materials of the layer formed to monotonously decrease the grating thickness is larger than at least one dispersion of the materials of the layer formed to monotonously increase the grating thickness.
Further, among the plurality of layers, if the diffractive optical element has an optical shape (optical path length) formed to monotonously decrease the grating thickness within one period, at least one dispersion of the materials of the layer formed to monotonously increase the grating thickness is larger than at least one dispersion of the materials of the layer formed to monotonously decrease the grating thickness.
Further, at least one of the plurality of layers is made of a plastic optical material or ultraviolet curable resin.
Further, the useful wavelength region is a visible spectrum.
Further, the plurality of layers are laminated on a base plate, and a layer which is in contact with the base plate, among the plurality of layers, is made of the same material as the material of the base plate.
Further, in the diffractive optical element according to one of the third and fourth aspects, the value of |xcfx86txc2x7xcex94t| mentioned above is equal to or less than m/8 of the useful wavelength.
An optical system according to the invention uses the diffractive optical element arranged in accordance with one of the aspects of the invention as mentioned above.
The diffractive optical element according to the invention is usable particularly for an image forming optical system and an observation optical system.
The arrangement according to the invention is applicable to optical apparatuses and electronic apparatuses having the optical systems mentioned above.
The above and other objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.